Multi-layer plastic articles are often used as containers to hold, food, beverages, pharmaceuticals, and nutraceuticals. Some multi-layer plastic articles are commonly made from materials such as polyethylene (PET) and polypropylene (PP). Articles made from PET and PP resist environmental degradation, and are reasonably durable, watertight, and economically produced. However, plastic materials such as PET and PP are gas (e.g., oxygen, nitrogen, etc.) permeable. For applications in which gas permeability is undesirable, for example, containers for food products, medicines and products that degrade upon gaseous exposure, a plastic article of PET or PP may include an interior layer of a barrier material or a gas scavenger material, such as ethylene vinyl alcohol (EVOH), between skin layers of PET or PP.
A common configuration of multi-layer plastic articles includes an interior or “core” layer, which is surrounded on substantially all sides by another material forming inner and outer layers. For example, see U.S. Pat. Nos. 5,914,138 and 6,187,241, both assigned to Kortec, Inc. The disclosures of both of these patents are incorporated herein by reference. For example, the interior (core) layer may be formed of EVOH and the surrounding plastic layer, including inner and outer layers, may be formed from PET or PP. This construction produces a sandwich structure in which the inner and outer layers (e.g., PET) form both the exterior and the interior surfaces of the article, and the interior (core) layer (e.g., EVOH) is sandwiched therebetween.
For example, the position of the core layer (e.g., barrier layer) in co-injection applications for gas barrier containers is critical in achieving gas barrier performance of the container. If the location of the barrier layer is too low, the container will have areas in the side wall where there is no barrier coverage, which results in high rates of gas permeation in a localized area. If the barrier layer is too high, there is a risk the barrier material will break through the skin flow front and end up on the outside surface of the part, which is undesirable.
Multi-layer plastic articles, e.g., articles with inner and outer layers of one material and one or more interior layers of another material, may be co-injection molded using a mold having a plurality of cavities. When co-injection molding a multilayer material having an interior (core) layer and surrounding inner and outer (skin) layers, the injection stream entering the cavity must initially contain the skin material before the core material is added to the stream. This is because the center of the stream, including the core material, flows faster than the edges or sides of the stream, including the skin material, that are in contact with cavity walls. The skin material needs a “head start” so that the core material does not reach the flow front of the skin material before the end of the injection, which could deposit core material on an outside of the article. However, if the skin material is given too great of a “head start,” the flow front of the core material does not substantially catch up with the flow front of the skin material during injection leaving a significant portion of the distal end of the article without any core layer.
Commonly, co-injection control systems are configured to initiate flow of the skin material from the nozzles into cavities simultaneously, and add core material to all flow simultaneously to all cavities simultaneously. In some systems the time delay between initiation of the flow of skin material and the addition of the core material is selected such that the core flow front will nearly catch up with, but not pass or break through the skin flow front during injection. In some systems configured for fold over of the internal core layer, the time delay between initiation of the flow of skin material and the addition of the core material is selected such that the core flow front will catch up with the skin flow front and fold over without breaking through the skin flow front.
Conventionally known injection molding techniques suitable for controlling placement and quality of materials in relatively thick multilayer articles (e.g., greater than 3 mm wall thickness) include thermally-balanced flow techniques and shooting pot techniques. In systems employing thermally-balanced techniques for producing relatively thick multilayer articles, the amount and timing of the introduction of the core materials and skin materials into the cavities are partially controlled by controlling the temperature of the skin material flow channels to a particular cavity relative to the skin material flow channels to the rest of the cavities, to achieve a desired flow rate into each cavity and volume of skin material flowing into each cavity before injection of the core material begins. By contrast, in systems employing shooting pot techniques for producing relatively thick-walled articles, shooting pots are used to determine the volume of core material and skin material fed into each cavity or group of cavities fed by that particular shooting pot. In injection molding apparatus for relatively thick-walled articles employing shooting pots, the volumetric stroke in one or more skin shooting pots will alter the position of the leading edge in one or more cavities, and changing the volumetric stroke in all skin shooting pots will alter the position in all cavities.